Stargate Atlantis is a spin-off of the series, Stargate SG-1. It follows an expedition that travels to the far Pegasus Galaxy in hopes of making contact with the Ancients, the race who created the stargates. Barring contact, they hope to find new technologies and information that will be a benefit to humanity.
There were 20 complete episodes in season one.
Short summaries -> (spoilers for entire season)